🐛 FIX: keep the hard line break after a literal backslash

[eeshsaxena]

markdown-it-py renders a literal backslash immediately before a two-space hard line break as a soft break with a stray space, rather than a hard break:

from markdown_it import MarkdownIt

MarkdownIt("commonmark").render("foo\\  \nbar")
# markdown-it-py: '<p>foo\\ \nbar</p>\n'
# markdown-it:    '<p>foo\\<br />\nbar</p>\n'

markdown-it's escape rule has a dedicated case for this: when a \ is followed by a space it emits just the backslash and leaves the space unconsumed, so the trailing two-space hard break is still detected by the newline rule (its escape.mjs even carries a comment to that effect). This port jumped straight to the generic escape branch, which consumed the space and left only one space before the newline, so the hard break was downgraded to a soft break.

This ports the missing space case so the output matches markdown-it. I added a regression test in test_misc.py, and the test_cmark_spec and test_port suites still pass.